Light-guide device with optical cutoff edge and corresponding production methods

ABSTRACT

A light-guide device includes a light guiding element (13) with a number of faces, including two parallel faces (26), for guiding light by internal reflection. A transparent optical element (19) has an interface surface for attachment to a coupling surface (14) of the light guiding element, and is configured such that light propagating within the transparent optical element passes through the interface surface and the coupling surface (14) so as to propagate within the light guiding element (13). A non-transparent coating (15) is applied to at least part of one or more faces of the light guiding element (13), defining an edge (17) adjacent to, or overlapping, the coupling surface (14) of the light guiding element (13). A quantity of transparent adhesive is deployed between the coupling surface and the interface surface so as to form an optically transmissive interface. An overspill region 31 of the adhesive extends to, and overlaps, the edge (17).

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to light-guide devices and, in particular,it concerns a light guide device which has a well-defined optical cutoffedge, and corresponding methods for producing such devices.

When manufacturing optical devices, it is often desirable to provide alight guiding device with a complex form in which different regions havesurfaces angled with selected non-rectilinear orientations. Examples ofsuch devices relevant to the present invention include, but are notlimited to, light guiding elements with coupling-in configurations, andregions of transition between light guiding elements with differenttransverse dimensions and/or different orientations.

FIGS. 1A and 1B illustrate two examples of devices which include a lightguiding element together with a coupling-in prism corresponding to FIGS.3 and 7, respectively, of PCT Publication No. WO 2015/162611. Referringto the original reference numerals of those drawings in parentheses,each of these devices has a light guiding element (20) with first andsecond parallel faces (26), and a coupling prism (44, 54) for attachmentto the light guiding element so as to provide a suitably angled inputsurface so that light can be introduced close to the normal to a surface(46, 58) of the prism and then enter the light guide at an angle desiredfor internal propagation of the light by internal reflection at surfaces(26).

SUMMARY OF THE INVENTION

The present invention is a light guide device and correspondingproduction methods.

According to the teachings of an embodiment of the present inventionthere is provided, an apparatus comprising: (a) a light guiding elementhaving a plurality of faces including first and second parallel faces,the light guiding element being configured for guiding light by internalreflection at the first and second parallel faces, one of the pluralityof faces providing a coupling surface; (b) a transparent optical elementhaving an interface surface for attachment to the coupling surface, thetransparent optical element being configured such that light propagatingwithin the transparent optical element passes through the interfacesurface and the coupling surface so as to propagate within the lightguiding element; (c) a non-transparent coating applied to at least partof at least one of the plurality of faces of the light guiding element,the coating defining an edge adjacent to, or overlapping, the couplingsurface of the light guiding element; and (d) a quantity of transparentadhesive deployed between the coupling surface and the interface surfaceso as to form an optically transmissive interface, the adhesiveextending to, and partially overlapping, the edge.

According to a further feature of an embodiment of the presentinvention, the coupling surface is provided on one of the first andsecond parallel faces.

According to a further feature of an embodiment of the presentinvention, the coating extends between the coupling surface and theinterface surface.

According to a further feature of an embodiment of the presentinvention, the coupling surface is inclined relative to the first andsecond parallel faces.

According to a further feature of an embodiment of the presentinvention, the coupling surface and the first parallel face meet at aground edge, and wherein the edge is non-overlapping with the groundedge.

According to a further feature of an embodiment of the presentinvention, the adhesive fills a recess formed between the ground edgeand the interface surface.

According to a further feature of an embodiment of the presentinvention, the coupling surface is obliquely inclined relative to thefirst and second parallel faces.

According to a further feature of an embodiment of the presentinvention, the coupling surface is perpendicular to the first and secondparallel faces.

According to a further feature of an embodiment of the presentinvention, the interface surface is larger than the coupling surface.

According to a further feature of an embodiment of the presentinvention, the coating is a metal coating.

According to a further feature of an embodiment of the presentinvention, the coating is a dielectric coating.

According to a further feature of an embodiment of the presentinvention, the transparent optical element is a coupling prismconfigured to provide an input surface oriented for input of light intothe light guiding element.

According to a further feature of an embodiment of the presentinvention, the edge defines an optical cutoff edge for light rayspassing from the transparent optical element into the light guidingelement.

There is also provided according to the teachings of an embodiment ofthe present invention, an apparatus comprising: (a) a light guidingelement having a plurality of faces including first and second parallelfaces, the light guiding element being configured for guiding light byinternal reflection at the first and second parallel faces, one of theplurality of faces providing a coupling surface; (b) a coupling prismhaving an interface surface adhered to the coupling surface and an inputsurface for input of light into the light guiding element; and (c) anon-transparent coating applied to at least part of the coupling surfaceof the light guiding element and extending between the light guidingelement and the coupling prism, the coating defining an optical cutoffedge for light passing from the coupling prism through the couplingsurface into the light guiding element.

According to a further feature of an embodiment of the presentinvention, the coupling prism is adhered to the coupling surface of thelight guiding element without adhesive.

There is also provided according to the teachings of an embodiment ofthe present invention, an apparatus comprising: (a) a light guidingelement having a plurality of faces including first and second parallelfaces, the light guiding element being configured for guiding light byinternal reflection at the first and second parallel faces, one of theplurality of faces providing a coupling surface; (b) a coupling prismhaving an interface surface for attachment to the coupling surface andan input surface for input of light into the light guiding element; and(c) a quantity of transparent adhesive deployed between the couplingsurface and the interface surface so as to form an opticallytransmissive interface, a part of the quantity of adhesive forming apartial filling of an angled recess between the light guiding elementand the coupling prism, wherein an air gap extends along one of thefaces of the light guiding element beneath the partial filling, the airgap terminating at an edge within the adhesive so as to define anoptical cutoff edge adjacent to the coupling surface of the lightguiding element.

There is also provided according to the teachings of an embodiment ofthe present invention, a method for producing an optical assembly, themethod comprising: (a) providing a light guiding element having aplurality of faces including first and second parallel faces, the lightguiding element being configured for guiding light by internalreflection at the first and second parallel faces; (b) applying acoating to at least part of at least one of the plurality of faces ofthe light guiding element; (c) grinding the light guiding element alonga plane intersecting the coating so as to simultaneously form a couplingsurface of the light guiding element and an edge of the coating; and (d)bonding to the coupling surface an interface surface of a transparentoptical element, the transparent optical element being configured suchthat light propagating within the transparent optical element passesthrough the interface surface and the coupling surface so as topropagate within the light guiding element, wherein the bonding isperformed by applying a quantity of a transparent adhesive between thecoupling surface and the interface surface, the quantity of adhesivebeing applied such that, when the coupling surface and the interfacesurface are pressed together, an excess of the transparent adhesiveoverlaps the edge of the coating.

According to a further feature of an embodiment of the presentinvention, the coating is a non-transparent coating such that the edgedefines an optical cutoff edge.

According to a further feature of an embodiment of the presentinvention, there is also provided a step of selectively removing thecoating so as to leave a cutoff edge defined by an edge of an air-gapformed in the transparent adhesive.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, withreference to the accompanying drawings, wherein:

FIGS. 1A and 1B, described above, correspond to FIGS. 3 and 7,respectively, of PCT Publication No. WO 2015/162611, illustrating lightguide devices to which the present invention may advantageously beapplied;

FIG. 1C is a schematic isometric view of a light guide device accordingto an aspect of the present invention in which context the presentinvention may also be advantageously applied;

FIGS. 2A, 2B and 2C are schematic enlarged cross-sectional partial viewsof a region of joining of two transparent optical elements illustrating,respectively, the junction without adhesive, with adhesive, and with aprotective layer overlapped by adhesive according to an embodiment ofthe present invention, each view illustrating various light ray pathsfor each case;

FIGS. 3A-3C are a sequence of schematic isometric views illustratingstages during the production of a light guide device according to anembodiment of the present invention, the views illustrating a lightguiding element after application of a coating, after grinding of acoupling surface, and after bonding of a coupling-in prism,respectively;

FIG. 3D is a schematic isometric view of a light guiding element afterapplication of a coating with a pre-formed edge according to analternative production sequence of certain embodiments of the presentinvention;

FIGS. 4A and 4B are schematic side views illustrating stages during theproduction of a further embodiment of the present invention illustratinga light guiding element after application of a coating, and afterbonding of a coupling-in prism, respectively;

FIG. 4C is a view similar to FIG. 4B illustrating the overall opticaleffect of a device resulting from the production process of FIGS. 4A and4B;

FIGS. 5A-5C are schematic side views illustrating stages during theproduction of a variant implementation of the device of FIG. 4C;

FIGS. 6A-6C are a sequence of schematic isometric views illustratingstages during the production of a light guide device according to afurther embodiment of the present invention, the views illustrating alight guiding element after application of a coating, after grinding ofa coupling surface, and after bonding of a coupling-in prism,respectively;

FIG. 6D is a view similar to FIG. 6C illustrating the overall opticaleffect of a device resulting from the production process of FIGS. 6A-6C;

FIG. 7A is a view similar to FIG. 2C illustrating a further variation ofthe present invention according to which a coating layer is removed toleave an air gap;

FIG. 7B is a view similar to FIG. 6D illustrating an implementation ofthat device using an air gap according to FIG. 7A;

FIG. 8A is an isometric view of a light guide device that is a furthervariant of the devices of FIGS. 1C and 3C, constructed and operativeaccording to the teachings of an embodiment of the present invention;

FIG. 8B is a schematic horizontal cross-sectional view taken through thedevice of FIG. 8A adjacent to the base of the light guiding element,illustrating propagation of an input image aperture along the lightguiding element; and

FIG. 9 is a schematic end view of a light guide device formed fromsuperposition of two light guiding elements, implemented according tothe teachings of an aspect of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is a light guide device and correspondingproduction methods.

The principles and operation of light guide devices according to thepresent invention may be better understood with reference to thedrawings and the accompanying description.

By way of introduction, the present invention relates to a wide range ofsituations in which two transparent elements are joined, and especially,although not exclusively, where external surfaces of the two elementsmeet at an angle or step. Joints between transparent elements of anoptical system present a number of challenges for practicalimplementation. Firstly, edges of blocks of transparent material are notperfectly sharp edges, and are modified (distorted) by some radius ofcurvature, often with some chipping or other defects. The extent of thisrounding typically varies depending on the properties of the materialsand the type of equipment used. FIG. 2A illustrates a schematic partialcross-sectional enlarged view of a region of joining of two transparentoptical elements 13 and 19, where an edge 32 of one of the elements 13after grinding exhibits a radius of curvature. This radius of curvatureresults in various scattering effects which reduce the overall qualityof the optical device. Thus, while rays R11 and R13 pass from opticalelement 19 to optical element 13 without scattering and propagate asintended along optical element 13, and ray R15 is excluded from opticalelement 13, rays R12 and R14 interact with various intermediate anglesof the surface due to the rounding of edge 32, resulting in scatteringof rays at angles other than those corresponding to the device design,with a consequent reduction in signal-to-noise for the overall device.

A further problem arises through the use of optical adhesives, asillustrated here in FIG. 2B. In order to ensure uniform opticalproperties at the interface, a sufficient quantity of optical adhesiveis interposed between the facing surfaces that, after pressing themtogether, the entire area of the facing surfaces to be joined is coveredwith adhesive. This typically leads to some outflow of excess adhesiveat the extremities of the surfaces to be joined, forming an overspillregion 31 which may be of arbitrary size and uncontrolled shape. Sincethe adhesive is transparent adhesive, and typically also index-matchedto the transparent optical elements, overspill region 31 definesadditional paths of optical rays which may result in scattering andunwanted angles of ray propagation within optical element 13. Thus, inthe example illustrated here, rays R23 and R24 pass from optical element19 to optical element 13 without scattering and propagate as intendedalong optical element 13, and ray R22 escapes without reaching opticalelement 13. However rays R21 and R25 interact with variousarbitrarily-angled surfaces of overspill region 31 of the adhesive,resulting in scattering of rays at angles other than those correspondingto the device design, with a consequent reduction in signal-to-noise forthe overall device.

According to one aspect of the present invention, as illustrated in FIG.2C, there is provided an optical device or apparatus which includes twotransparent optical elements, typically a light guiding element 13 witha plurality of faces including first and second parallel faces 26 sothat light can be guided within element 13 by internal reflection atfirst and second parallel faces 26. The second transparent opticalelement 19 has an interface surface for attachment to a coupling surfaceof light guiding element 13. A non-transparent (opaque) coating 15 isapplied to at least part of at least one of the faces of light guidingelement 13, in this case, one of the parallel faces 26. Coating 15 ispreferably chosen to provide (or maintain) reflective properties forinternal reflection at the surface of the light guiding element, andpreferably defines an edge 17 adjacent to, or in some cases describedbelow overlapping, the coupling surface of light guiding element 13. Aquantity of transparent adhesive is deployed between the couplingsurface and the interface surface so as to form an opticallytransmissive interface, the adhesive forming an overspill region 31which extends to, and partially overlaps, the edge 17.

As seen in FIG. 2C, the presence of coating 15 according to thisexemplary implementation significantly enhances the optical propertiesof the device. Firstly, due to the presence of coating 15 on the surfaceof light guiding element 13, the adhesive of overspill region 31 doesnot compromise the internal reflection properties of the light guidingelement 13, such that ray R31 is internally reflected within the lightguiding element and propagates correctly within the light guidingelement. Additionally, edge 17 here serves as an optical cutoff edge,cleanly delineating between rays R31, R33 and R34 which enter the lightguiding element undistorted, and rays R32 and R35 which are excludedtherefrom. Any stray light that is reflected by irregular surfaces ofthe adhesive overspill region 31, such as ray R35, hits the outersurface of coating 15 and is excluded from the light guiding element.

Coating 15 may be formed from any material suitable for application to asurface of an optical element and which provides the requiredlight-blocking properties and provides internally reflective propertiesof the transparent optical elements. Examples include, but are notlimited to, various metal coatings and various dielectric coatings. Inone particularly preferred but non-limiting example, a coating of silverprotected by a thin sealing layer to prevent oxidation has been found tobe particularly effective and suitable for this application.

The present invention is applicable to a wide range of applications inwhich two transparent optical elements are joined together. A subset ofapplications of particular importance relates to devices in which lightis introduced into a light guiding element 13, either from another lightguiding element or from a coupling-in configuration 19. Suchapplications can employ a number of different geometries of attachment,which employ attachment of optical element 19 to various differentsurfaces of the light guiding element 13, as illustrated in FIGS. 1A-1C.In FIG. 1A, a coupling-in prism is attached to one of the major parallelsurfaces of the light guiding element, while in FIG. 1B, attachment of aprism occurs at an obliquely angled coupling-in surface. Attachment mayalso occur at an end surface perpendicular to the major surfaces of alight guiding element, such as is illustrated in the novel configurationof FIG. 1C, which relates to a rectangular light guiding element withtwo pairs of parallel surfaces. Further examples of an implementation ofthe present invention for each of these geometries may be found below.

The sequence of operations to produce optical devices according to thepresent invention may vary considerably according to the particulardesign employed. FIGS. 3A-3C illustrate a sequence of stages ofproduction, corresponding to a method of one particularly preferred, butnon-limiting, aspect of the present invention. In this case, productionof an optical assembly includes a step of applying a coating 15 to atleast part of at least one face of a light guiding element 13, which maybe a slab-type guiding element with two major parallel faces, or may bea rectangular (including square) light guiding element with two pairs ofparallel faces for guiding light through four-fold internal reflection,as shown in FIG. 3A. According to the particularly preferred sequenceshown here, light guiding element 13 is then ground along a planeintersecting the coating so as to form simultaneously an end couplingsurface 14 and an edge 17 of coating 15, as shown in FIG. 3B. Aninterface surface of a transparent optical element, such as acoupling-in prism 19, is then bonded to coupling surface 14, so thatlight propagating within the transparent optical element can passthrough the interface surface and the coupling surface so as topropagate within the light guiding element. Bonding of prism 19 to lightguiding element 13 is performed by applying a quantity of a transparentadhesive between the coupling surface and the interface surface. Theadhesive is applied such that, when the coupling surface and theinterface surface are pressed together, an excess of the transparentadhesive overlaps edge 17 of coating 15, thereby generating a finalconfiguration which is analogous to that described above with referenceto FIG. 2C, where the coating prevents the adhesive from adverselyaffecting the light guiding element properties, and where edge 17provides a well-defined optical cutoff edge delineating what beams doand do not enter the light guiding element. This approach may be used toadvantage for coupling surfaces at any desired angle, includingorthogonal coupling surfaces such as that of FIG. 1C and obliquecoupling surfaces such as that of FIGS. 3B and 3C.

The simultaneous forming of coupling surface 14 and edge 17 in agrinding/polishing process is considered advantageous in that it ensurescorrect positioning of edge 17 relative to coupling surface 14,typically just beyond any non-planar edge effects occurring at theextremity of the coupling surface, such as shown in FIG. 2C, and avoidsthe need for formation of a sharp edge during the coating process. As aresult of this process, coupling surface 14 and one of the parallelfaces of the light guiding element meet at a ground edge, and edge 17 ofthe coating is in non-overlapping relation with the ground edge. A“ground edge” in this context refers to the entire region over which anangular transition occurs which deviates from flat surface.

Alternatively, as illustrated in FIG. 3D, a region of coating 15 may beapplied to a light guiding element 13 in such a manner as to define anedge 17 during the application of the coating. Techniques for applyingthe coatings of the present invention in general, and in particular, forselective application of a layer so as to form a well-defined edge, areknown in the art, and can be chosen according to the type of coatingused, and the corresponding application technique employed. For example,a protected silver coating or a dielectric coating can be applied byconventional deposition techniques, such as sputter coating or wetchemical deposition, and the shape of the coating can be defined byvarious conventional techniques including, but not limited to,lithographic techniques defining a pattern of photoresist, andmechanical masking such as by application of adhesive tape. Applicationof the coating with a well-defined edge defined during the applicationprocess is particularly useful in applications in which anothertransparent optical element is to be bonded to the light guiding elementon one of the major parallel surfaces, such as in the configuration ofFIG. IA. An example of such an application will be described below withreference to FIGS. 4A-4C.

The coatings of the present invention may be applied on one or moresurface of one or both of the optical elements to be bonded, and may beapplied to either the entirety of the surface(s) or, more preferably, toonly a part of the surface(s) which is sufficiently close to the bondingregion to be needed for its adhesive-protection properties. Thus, inmany implementations, the total area of the coating in the finalassembled device is less than half the total surface area of thecorresponding surface, and in certain cases, is less than the total areaof the bonded surfaces.

In certain applications, coating on only one side, or a subset of thefaces, may be sufficient. For example, where two components are to becoupled so that certain surfaces will be flush after coupling, it may bepossible to remove excess adhesive effectively after bonding byperforming a further polishing step which is effective to resurface thetwo components in a common plane.

Turning now to FIGS. 4A-4C, these illustrate an implementation of thepresent invention in which the coupling surface is provided on one ofthe major parallel faces defining the light guide. In this case, coating15 is advantageously applied so as to define edge 17, most preferably ina location which is to be overlapped by the transparent optical element19 after attachment (FIG. 4B). After attachment of the transparentoptical element 19, the coating is seen to extend between the couplingsurface and the interface surface. As a result, any overspill region 31of adhesive, as well as the ground edge of coupling prism 19, falloutside the optical cutoff edge 17 such that the adhesive overspill andany imperfections of the coupling prism edge do not adversely affect theoptical properties of the device.

FIG. 4C schematically depicts the overall optical properties of theassembled components. As in a number of other particularly preferredimplementations illustrated here, transparent optical element 19 is acoupling prism configured to provide an input surface 18 oriented forinput of light into light guiding element 13. Specifically, for a lightguiding element which is implemented as part of a system in which lightpropagates within the element within a given range of angles, inputsurface 18 may advantageously be oriented roughly perpendicular to theinput ray directions, thereby minimizing distortions. Additionally,employing edge 17 as an optical cutoff, this configurations can beutilized for “filling” the light guiding element with rays of an image,where a slightly oversized image aperture is “trimmed” by an opticalcutoff edge to ensure that the image (and its reflected conjugates) arepresent at all locations within the light guiding element. For thispurpose, edge 17 does not necessarily have to be a straight edge, butshould be a cleanly defined edge of whatever shape is desired. Variousarrangements for filling of a light guiding element with an image aredescribed in PCT Patent Publication No. WO 2015/162611 for light guidingelements with one pair of parallel faces, and in co-pending PCT PatentApplication No. PCT/IL2017/051028 (which was unpublished on the filingdate of this application) for light guiding elements with two pairs ofparallel faces. In each of those configurations, the optical cutoff edgeserving to trim the in-coupled images can advantageously be implementedaccording to the teachings of the present invention. It will be notedthat the trimming edge thickness is defined by the thickness of thecoating layer, which is thin, and will generate minimal scattering.

FIGS. 5A-5C illustrate a modified production sequence and final form ofan optical assembly functionally similar to that of FIGS. 4A-4C, but inwhich the optical geometry is enhanced. In this case, light guidingelement 13 is coated with coating 15, as in FIG. 4A. In the next step(FIG. 5B), a rectangular prism 19 is glued on one of the primaryparallel faces of light guide element 13, partly covering the coating15. The use of a rectangular prism during assembly facilitates effectivepressing together of the prism to the light guiding element, therebyachieve better bonding. The bonded prism and light guiding element arethen polished along the dashed line to generate an enhanced devicegeometry, as depicted in FIG. 5C.

Parenthetically, although described primarily in relation to devices inwhich optical elements are bonded by use of optical adhesive, it shouldbe noted that certain examples of the present invention can beimplemented without adhesive, where alternative bonding techniques areused. The structures of FIGS. 4A-4C and 5A-5C are examples of structuressuitable for such an implementation, where the coupling surface of lightguiding element 13 and the interface surface of transparent opticalelement 19 are carefully prepared to a high degree of planarity and thenjoined by glueless contact bonding (“direct bonding”) by bringing thesurfaces into direct contact. In such cases, there is no problem ofadhesive overspill. However, the technique of providing a coating withan optical cutoff edge extending between the components ensures a highquality optical cutoff, independent of any imperfections in the edge ofthe coupling prism.

Turning now to FIGS. 6A-6D, these illustrate an exemplary non-limitingprocess for implementing the present invention in the context of ageometry similar to that of FIG. 1B. In this example, light guidingelement 13 is first coated with coating 15 as shown in 6A. The end ofthe light guiding element is then polished to form the coupling surface,and simultaneously shortens the coated region so as to generate trimmingedge 17, as shown in FIG. 6B. In FIG. 6C, prism 19 is bonded to thecoupling surface of the light guiding element 13, with any adhesiveoverspill 31 overlying edge 17 and part of coating 15. In this case,optionally, the interface surface of prism 19 may advantageously belarger than the coupling surface of the light guiding element. FIG. 6Ddepicts schematically the overall optical structure of the combinedassembly, in which edge 17 provides an optical cutoff, and the opticalproperties are insensitive to imperfections of the angled edges of theelements.

Turning now to FIGS. 7A and 7B, there is shown a further variantimplementation of the present invention. In this case, instead ofemploying an opaque coating to define a cutoff edge, a removable coating15 is used to protect the light guiding element surfaces duringapplication of adhesive, and to define cutoff edge 17. After the bondingis complete and the adhesive has solidified, the coating is selectivelyremoved so as to leave a cutoff edge defined by an edge of an air-gap 34formed in the transparent adhesive.

In this case, the coating/layer does not need to have any particularoptical requirements, and is only present during application of adhesiveto attach the coupling-in prism 19. The air-gap 34 is generated afterthe material of the layer (such as photoresist or wax) has been removed.The optical properties of this configuration including the behavior ofthe various rays is analogous to the rays illustrated in FIG. 2C, exceptthat reflectance of ray R21 is now by total internal reflection (TIR) ofthe light guiding element (instead of layer reflectivity) and thereflectance of R25 is by TIR within the adhesive (instead of from anouter surface of the coating). Trimming edge 17 is now determined by theedge of the air-gap, after which the optical path is continuous. Theoverall optical properties of the device are depicted schematically inFIG. 7B.

The present invention may be implemented in the contexts of opticalelements that are bonded at surfaces of any orientation, including atcoupling surfaces that are perpendicular to an extensional direction ofa light guiding element, such as was illustrated above in FIG. 1C. Asmentioned, the invention is also applicable to light guiding elementswith two pairs of parallel surface within which rays propagate byfour-fold reflection, referred to as a “2D waveguide.”

In certain cases, protective layer coatings may be used to advantage onselected surfaces of two elements being bonded. Thus, in FIGS. 8A and8B, it may be preferably to apply a protective coating on both sides oflight guiding element 13 in order to reduce non uniformity that can becaused by a discontinuity between prism 19 and light guiding element 13(region 37 in FIG. 8A). The coating is shown on both sides of lightguiding element 13 as 80 f 1 and 80 f 2. By introducing a protectivecoating layer all around the end of light guiding element 13 (including80 f 1, 80 f 2 and the other two orthogonal sides) the guidance of thewaveguide will be protected from glue spillover or edge non-uniformitiesfrom all sides. If the step between prism 19 and light guiding element13 near 80 f 2 is small (or doesn't exist) then it is beneficial to alsoprotect the reflectance of the prism 19 from any glue spillover byprotecting coating 80 f 3 as well.

Turning finally to FIG. 9, although illustrated herein primarily in thecontext of a coupling-in prism attached to a light guiding element, thepresent invention can equally be applied to other applications such as,for example, where a first light guiding element (or “waveguide”) feedsinto another. FIG. 9 illustrates one such example in which a rectangular(2D) light guiding element 10 feeds into a slab-type (1D) light guidingelement 20. This configuration corresponds to one of a number of suchoptions described in the aforementioned co-pending PCT PatentApplication No. PCT/IL2017/051028 (which was unpublished on the filingdate of this application), and the invention can equally be applied tothe various variant implementations, with or without a coupling prism,described therein.

Waveguide 10 is slightly larger than waveguide 20 in order to enablecomplete illumination (filling) of waveguide 20. The glue 2110 canspillover either or both of waveguides 20 and 10. Particularly preferredimplementations of the present invention thus provide protective coatinglayers on both waveguides, as illustrated at 40 f 1, 40 f 2, 40 f 3 and40 f 4.

To the extent that the appended claims have been drafted withoutmultiple dependencies, this has been done only to accommodate formalrequirements in jurisdictions which do not allow such multipledependencies. It should be noted that all possible combinations offeatures which would be implied by rendering the claims multiplydependent are explicitly envisaged and should be considered part of theinvention.

It will be appreciated that the above descriptions are intended only toserve as examples, and that many other embodiments are possible withinthe scope of the present invention as defined in the appended claims.

What is claimed is:
 1. An apparatus comprising: (a) a light guidingelement having a plurality of faces including first and second parallelfaces, said light guiding element being configured for guiding light byinternal reflection at said first and second parallel faces, one of saidplurality of faces providing a coupling surface; (b) a transparentoptical element having an interface surface for attachment to saidcoupling surface, said transparent optical element being configured suchthat light propagating within said transparent optical element passesthrough said interface surface and said coupling surface so as topropagate within the light guiding element; (c) a non-transparentcoating applied to at least part of at least one of said plurality offaces of said light guiding element, said coating defining an edgeadjacent to, or overlapping, said coupling surface of said light guidingelement; and (d) a quantity of transparent adhesive deployed betweensaid coupling surface and said interface surface so as to form anoptically transmissive interface, said adhesive extending to, andoverlapping, said edge.
 2. The apparatus of claim 1, wherein saidcoupling surface is provided on one of said first and second parallelfaces.
 3. The apparatus of claim 2, wherein said coating extends betweensaid coupling surface and said interface surface.
 4. The apparatus ofclaim 1, wherein said coupling surface is inclined relative to saidfirst and second parallel faces.
 5. The apparatus of claim 4, whereinsaid coupling surface and said first parallel face meet at a groundedge, and wherein said edge is non-overlapping with said ground edge. 6.The apparatus of claim 5, wherein said adhesive fills a recess formedbetween said ground edge and said interface surface.
 7. The apparatus ofclaim 4, wherein said coupling surface is obliquely inclined relative tosaid first and second parallel faces.
 8. The apparatus of claim 4,wherein said coupling surface is perpendicular to said first and secondparallel faces.
 9. The apparatus of claim 1, wherein said interfacesurface is larger than said coupling surface.
 10. The apparatus of claim1, wherein said coating is a metal coating.
 11. The apparatus of claim1, wherein said coating is a dielectric coating.
 12. The apparatus ofclaim 1, wherein said transparent optical element is a coupling prismconfigured to provide an input surface oriented for input of light intosaid light guiding element.
 13. The apparatus of claim 1, wherein saidedge defines an optical cutoff edge for light rays passing from saidtransparent optical element into said light guiding element.
 14. Anapparatus comprising: (a) a light guiding element having a plurality offaces including first and second parallel faces, said light guidingelement being configured for guiding light by internal reflection atsaid first and second parallel faces, one of said plurality of facesproviding a coupling surface; (b) a coupling prism having an interfacesurface adhered to said coupling surface and an input surface for inputof light into the light guiding element; and (c) a non-transparentcoating applied to at least part of said coupling surface of said lightguiding element and extending between said light guiding element andsaid coupling prism, said coating defining an optical cutoff edge forlight passing from said coupling prism through said coupling surfaceinto said light guiding element.
 15. The apparatus of claim 14, whereinsaid coupling prism is adhered to said coupling surface of said lightguiding element without adhesive.
 16. An apparatus comprising: (a) alight guiding element having a plurality of faces including first andsecond parallel faces, said light guiding element being configured forguiding light by internal reflection at said first and second parallelfaces, one of said plurality of faces providing a coupling surface; (b)a coupling prism having an interface surface for attachment to saidcoupling surface and an input surface for input of light into the lightguiding element; and (c) a quantity of transparent adhesive deployedbetween said coupling surface and said interface surface so as to forman optically transmissive interface, a part of said quantity of adhesiveforming a partial filling of an angled recess between said light guidingelement and said coupling prism, wherein an air gap extends along one ofsaid faces of said light guiding element beneath said partial filling,said air gap terminating at an edge within said adhesive so as to definean optical cutoff edge adjacent to said coupling surface of said lightguiding element.
 17. A method for producing an optical assembly, themethod comprising: (a) providing a light guiding element having aplurality of faces including first and second parallel faces, said lightguiding element being configured for guiding light by internalreflection at said first and second parallel faces; (b) applying acoating to at least part of at least one of said plurality of faces ofsaid light guiding element; (c) grinding said light guiding elementalong a plane intersecting said coating so as to simultaneously form acoupling surface of said light guiding element and an edge of saidcoating; and (d) bonding to said coupling surface an interface surfaceof a transparent optical element, the transparent optical element beingconfigured such that light propagating within the transparent opticalelement passes through the interface surface and the coupling surface soas to propagate within the light guiding element, wherein said bondingis performed by applying a quantity of a transparent adhesive betweensaid coupling surface and said interface surface, said quantity ofadhesive being applied such that, when said coupling surface and saidinterface surface are pressed together, an excess of said transparentadhesive overlaps said edge of said coating.
 18. The method of claim 17,wherein said coating is a non-transparent coating such that said edgedefines an optical cutoff edge.
 19. The method of claim 17, furthercomprising a step of selectively removing said coating so as to leave acutoff edge defined by an edge of an air-gap formed in said transparentadhesive.